Formation of Sedimentary Rocks
- weathering produces sediments from rock and mineral fragments
- less stable minerals chemically broken down and more resistant grains are broken off as
smaller grains
- erosion carrys and deposition settles in a location
- fast- moving water carry large particles; slow moving settle out last
- wind can only move small grains ex. sand dunes
- glaciers move all with ease
Lithification
- most sediments deposited on earth in low areas = valleys and ocean basins
- more sediment = bottom layers increase in pressure and temp.
- compaction - lithification starts here
- weight of overlying sediments forces grains closer
sand to sand usually form a framework that leaves spaces - groundwater, oil, and natural gas can
be found in these spaces
- cementation - mineral growth glues sediments together into a solid rock; ex. iron oxide and
calcite grow between grains as dissolved minerals come out of groundwater
Sedimentary Features
- characteristics that help geologists understand how and where the rocks formed
Bedding
- primary - horizontal layering
- results from the way sediment settled out of wind or water
- mm - several meters
- 2 types - depend on method of transport
Cross - Bedding
- inclined layers of sediment are deposited across a horizontal surface
ex. - sandy beaches and migrating dunes
Graded Bedding
- particle sizes becoame heavier and coarser toward bottom layers
- found in marine sedimentary rock from underwater landslides
Sorting and Rounding
- some sediments have jagged (broken apart) and rounded (transported) edges
- rounding depends on how far sediment traveled and how hard it is before breaking
- ex. quartz sand on beaches = round and carbonate ( softer due to seashells and calcite) sand is
more angular
Evidence of Past Life
- fossils - preserved remains
- sometimes buried before decomposition
- during lithification - parts of organism can be replaced by minerals and turned into rock
Types of Sedimentary Rocks
- classified by their formation
- clastic (broken) sedimentary rocks - most common; formed from loose sediments that build-up
on earth's surface
- further classification based on particles size and mineral content
Coarse-grained rocks
- gravel- sized rocks and mineral fragments
- ex. Conglomerates have gravel-sized, rounded particles
- transported in high energy flows of water
- Breccias - angular, gravel sized - no time to round; transported across short distances and
deposited close to source
Medium-grained rocks
- sand-sized and mineral fragments
- sandstone - high porosity (% of open spaces between grains)
- loose sand up to 40%; fluids are able to move through
Fine-grained rocks
- silt and clay sized particles
- ex. siltstone and shale
- found in swamps and ponds; slow moving water
- settle to bottom - build up in horizontal layers
- low porosity
- forms barriers
Chemical Sedimentary Rock
- evaporites - layers of rock as a result of high concentration of dissolved minerals that
precipitate out of solution and settle to bottom
- form in arid areasdue to low water flow
Biochemical Sedimentary Rock
- formed from remains of once living things
- ex. limestone (shallow water) composed of Calcite
- shells settle to bottom of ocean and form thick layers of carbonate sediment